Why is Master Chief Collection always updating?

The reason why youre seeing them every few weeks is because every few weeks/months now they have been adding a new Halo game to the PC version and simultaneously have been adding content/features (in the form of the new seasons) that coincide with each release.

How many GB is the Master Chief Collection 2020?

MCC will receive a 24.37GB update with the full install being 99.4GB.

How many GB is Halo 3 PC?

55 GB Storage: 55 GB available space. Additional Notes: 30 FPS on performance settings at 1920x1080.